Set accelerators are used to speed up the setting of hydraulic cement mixes so that the work can be finished quickly, such as for example when it is necessary to work with hydraulic cement compositions at freezing or near-freezing temperatures. The rate of hydration of Portland cement is very dependent on temperature, such that, at lower temperatures, Portland cement compositions will often set at a rate slower than desired unless the setting process is accelerated.
A variety of techniques have been used to accelerate the setting of hydraulic cement mixes in order to overcome the extended time required for hardening of such mixes under a variety of application conditions. Some techniques for accelerating the set are: increasing the proportion of cement in the mix, heating the mix, and using chemical admixtures that act, catalytically or otherwise., on the components of the mix to increase the rate at which the cement paste sets.
Several chemical set accelerators for hydraulic cement-based compositions are well known. They include alkali hydroxides, silicates, fluorosilicates, calcium formate, sodium chloride, calcium chloride, calcium nitrate and calcium nitrite. Calcium chloride is widely used because it is easy and inexpensive to produce and its effects on hydraulic cement are predictable and well-documented. However, like sodium chloride, it has the disadvantage of corroding reinforcing steel embedded in or in contact with the calcium chloride-containing concrete. Other drawbacks of calcium chloride include reduced compressive strength at later ages, reduced response to air entrainment, and blotching of hardened concrete surfaces (Concrete Admixtures, Dodson: Van Nostrand Reinhold, 1990).
Other set accelerating agents which do not have the disadvantage of corroding metal are also known. However, these agents exhibit only a small degree of influence on the rate of hardening and do not provide pronounced improvement in the initial low temperature set time of cement compositions. Examples of such non-corrosive agents include urea-formaldehyde condensates, melamine-formaldehyde condensates, as well as the inorganic salts of formic acid, thiocyanic acid, nitric acid, and carbonic acid. Thus, commonly used chloride-free set accelerators for Portland cement concretes are based on calcium nitrate and an organic component typically consisting of an amine-aldehyde adduct. It is desirable to also include nitrites in such formulations in order to provide corrosion inhibition and strength gain, but nitrites react with amine-aldehyde adducts to form nitrosamines, which are highly carcinogenic.
It is thus an object of the present invention to provide new set-accelerating admixtures which reduce the set time of hydraulic cement compositions without inducing or promoting corrosion of steel reinforcement.
It is a further object of this invention to provide non-corrosive set-accelerating admixtures which function well at low temperature in order to permit construction of concrete structures to proceed at an acceptable pace during cold climate conditions.
It is a still further object of the present invention to provide non-corrosive set-accelerating admixtures which can be used in combination with other admixtures which are known to cause retardation of the initial set and early strength properties of hydraulic cement-based compositions, in order to overcome the retardation effect of such other admixtures.
Further objects of the invention are to provide methods for preparing such cement compositions and to provide methods for reducing the set time of hydraulic cement compositions.